1. Field of the Invention
The present invention relates to an improved handover control method and apparatus, and more particularly, to a handover control method and apparatus for efficiently buffering packets in a Worldwide Interoperability for Microwave Access (WIMAX) network.
2. Description of the Related Art
As the demands of users for various large-capacity multimedia services increase, broadband wireless access networks are recently being implemented. Mobile Worldwide Interoperability for Microwave Access (WIMAX) serves as a wireless network capable of providing users with a broadband service of several tens Mb/s (Megabit per Second) on the move. The mobile Microwave Access (WIMAX) may easily provide large-capacity, high-speed services for Internet data, voice data, image data, and the like. In general, the mobile Microwave Access (WIMAX) may be configured with a radio access station (RAS) for performing a function of a base station, an access control router (ACR) for performing a function of a base station controller, and the like. In the mobile Microwave Access (WIMAX), the users receive a seamless service through a handover between a plurality of Radio access stations (RASs). In general, an access service network (ASN) anchored mobility handover technique for the handover is applied to the mobile Microwave Access (WIMAX). To apply the Access service network (ASN) anchored mobility handover, a method for preventing the loss of data in a handover process is required. That is, the users should be able to seamlessly receive data after accessing a target Radio access station (T-RAS) while receiving the data through a serving Radio access station (S-RAS).
A Microwave Access (WIMAX) network constructed for contemporary practice for an Access service network (ASN) anchored mobility handover may be configured with a mobile station (MS), a serving radio access station (S-RAS), an Access control router (ACR), a target radio access station (T-RAS), and the like. The mobile station (MS) sends a MOB-MSHO-REQ message to the serving radio access station (S-RAS) so as to request a handover preparation step. Upon receiving the MOB-MSHO-REQ message from the mobile station (MS), the serving Radio access station (S-RAS) sends an HO-Request message to the Access control router (ACR). The Access control router (ACR) delivers the HO-Request message to the target Radio access station (T-RAS) capable of performing the handover for the mobile station (MS).
In this regard, the target Radio access station (T-RAS) sends a DP-Pre-Reg-Req message to the Access control router (ACR) and then receives a DP-Pre-Reg-Rsp message from the Access control router (ACR) in response thereto. The target Radio access station (T-RAS) receives the DP-Pre-Reg-Rsp message from the Access control router (ACR) and delivers an HO-Response message to the Access control router (ACR). Through this process, the target Radio access station (T-RAS) establishes a pre-path with the Access control router (ACR) for sending a packet to be buffered.
After receiving the HO-Response message from the target Radio access station (T-RAS), the Access control router (ACR) independently buffers a packet to be sent to the mobile station (MS). The Access control router (ACR) sends the HO-Response message to the serving Radio access station (S-RAS). The serving Radio access station (S-RAS) sends an MOB-BSHO-RSP message to the mobile station (MS).
The mobile station (MS) moves to a predefined area and initiates a substantial handover process by sending an MOB-HO-IND message to the serving Radio access station (S-RAS). The serving Radio access station (S-RAS) sends an HO-Confirm message to the Access control router (ACR). The Access control router (ACR) delivers the HO-Confirm message to the target Radio access station (T-RAS) for performing the handover for the mobile station (MS).
Upon receiving the HO-Confirm message, the target Radio access station (T-RAS) sends a DP-Reg-Req message to the Access control router (ACR). The DP-Reg-Req message includes a sequence number of the last packet received by the mobile station (MS). The sequence number corresponds to information to be delivered to the target Radio access station (T-RAS) through a transmission of the MOB-HO-IND message from the mobile station (MS) and the HO-Confirm message and the like.
The Access control router (ACR) returns a DP-Reg-Rsp message serving as a response indicating that the DP-Reg-Req message has been received to the target Radio access station (T-RAS). Through this process, the target Radio access station (T-RAS) and the Access control router (ACR) generate a substantial data path for sending a buffered packet. Thereafter, the Access control router (ACR) performs a process for delivering a packet subsequent to that of a sequence number included in the DP-Reg-Req message among packets buffered through the established data path.
Upon sending the DP-Pre-Reg-Rsp message, the Access control router (ACR) performs an operation for sending bi-cast data to a plurality of target radio access stations. That is, the Access control router (ACR) copies a packet delivered from a core network (not shown), and then sends the packet to all of the serving Radio access station (S-RAS) and the target radio access stations to which the bi-casting path is established.
A process for sending an HO-Response message from the target Radio access station (T-RAS) to the Access control router (ACR), a process for sending the HO-Response message from the Access control router (ACR) to the serving Radio access station (S-RAS), and a process for delivering an MOB-BSHO-RSP message from the serving Radio access station (S-RAS) to the mobile station (MS) are the same.
Then, the mobile station (MS) delivers an MOB-HO-IND message to the serving Radio access station (S-RAS) so as to initiate the handover, and an HO-Confirm message from the serving Radio access station (S-RAS) passes through the Access control router (ACR) and is delivered to the target Radio access station (T-RAS). As described above, the HO-Confirm message includes sequence number information of the last packet received by the mobile station (MS).
The target Radio access station (T-RAS) exchanges a DP-Reg-Req message and a DP-Reg-Rsp message with the Access control router (ACR). After the handover process for the mobile station (MS) is completed in the above-described process, the target Radio access station (T-RAS) provides the mobile station (MS) with a packet subsequent to that of the last sequence number received by the mobile station (MS) among buffered packets.
When the above-described handover process is used, the following problems occur. First, network resources may be wasted in the handover process since pre-paths should be established on a target radio access station-by-target radio access station basis in the handover preparation process. Another problem is that an unnecessary buffering is performed and the processing capacity of the Access control router (ACR) is limited since the Access control router (ACR) starts in advance a buffering operation.
There is also a vital problem in that a bi-casting path may be generated to only one target Radio access station (T-RAS) according to the so far known mobile Microwave Access (WIMAX) technologies. The bi-casting path should be established between an Access control router (ACR) and a plurality of target radio access stations in which the handover for the mobile station (MS) is possible. There is a problem in that this technology is not yet implemented. Radio access station (RAS) resources are wasted since the target Radio access station (T-RAS) buffers a packet to be discarded.
Moreover, the contemporary handover methods have a drawback in that a process for providing the target Radio access station (T-RAS) with a sequence number of the last packet, received by the mobile station (MS), is necessarily required.